In recent years, the prevalence of digital cameras has been significant. With a compact digital camera for beginners to photography, it is desirable to be able to take photos of roughly a certain quality regardless of the user's skill. Therefore, focusing is normally performed automatically, and there is a need to shorten the focusing operation period as much as possible so that a photo opportunity is not missed.
Many compact digital cameras do not have an optical viewfinder, and in the focusing operation period as well, an imaging subject continues to be viewed on a liquid crystal display unit arranged on the back face of the digital camera body. Therefore, in the focusing operation period as well, it is necessary to display an image of the imaging subject on the display unit without causing a sense of unpleasantness.
There are mainly two automatic focusing operation methods, namely an active method and a passive method. An active method is a method in which the distance to an imaging subject is measured by irradiating an imaging subject with infrared radiation or ultrasonic waves and receiving infrared radiation or ultrasonic waves that have reflected off of the imaging subject. Active methods often are employed in compact silver salt cameras.
On the other hand, a passive method is a method in which the distance to an imaging subject is measured based on images captured by an optical system. Passive methods further can be divided into phase difference detection methods and contrast detection methods. A phase difference detection method often is employed in single lens reflexive cameras, regardless of whether the camera is a silver salt camera or digital camera. On the other hand, a contrast detection method is often employed in compact digital cameras.
In a contrast detection method, images are obtained from an imaging element while gradually moving a focus lens in an optical system in the optical axis direction, and the focal position is set to the position of the focus lens where the contrast of the obtained image is maximal. Contrast generally is evaluated based on a high frequency component in an image obtained from an imaging element.
Since the evaluation of the high frequency component is performed on a frame-by-frame basis, shortening the focusing operation period requires raising the read frame rate of the imaging element. However, when the read frame rate of the imaging element is raised, the time available for processing output image signals of the imaging element becomes shorter. Therefore, the image displayed on the display unit is frozen in the focusing operation period.
A digital camera that solves the above problem has been proposed (see Patent Document 1). FIG. 9 is a conceptual diagram of a display unit in the focusing operation period of the digital camera according to Patent Document 1. Since the imaging subject that the user is focusing on often exists in the center of the screen, only the center portion of the imaging element is used in the focusing period.
A cutout area 501 necessary for the focusing operation is an area in which an image (through-the-lens image) is displayed based on an image signal obtained by reading the image signals of all the pixels. On the other hand, non-cutout areas 502a and 502b that are not used in the focusing operation are areas in which signals output from the imaging element are not processed and an image is freeze-displayed in these areas based on images signals that have been output from the imaging element immediately before the focusing operation period. Accordingly, even if the read frame rate of the imaging element is raised, only image signals output from the imaging element pertaining to the cutout area 501 are processed, thereby enabling performing through-the-lens display in the cutout area 501.
Patent Document 1: JP 2002-300457A